If you have been playing competitive games with some kind of competitive ranking system, you are probably aware of how the MMR system works. However, some gamers are new to competitive video games and are just starting to play ranked video games. The MMR system is a ranking system for competitive games, that simply gives you a rank according to your skill level. Your skill level is measured depending on a number of factors like the kills you got etc., which make up Match Making Rating or MMR.
Along with the normal MMR system that most games have, Dota 2 also has an unranked MMR system or a hidden MMR. If you play unranked matches, you will be getting points or losing points for a hidden MMR depending on your performance. In other words, even if you play unranked matches, you start getting matched with better and better players as you perform better and better. If you perform worse and lose more matches, you will start getting matched with players that are of a lower skill level. Because this MMR is hidden, there is no accurate way of predicting it. Another thing that the hidden MMR system does is that as you play several unranked matches and develop a hidden MMR, the game will take that into account as you play your first ever Ranked match. After that, the game will add or subtract MMR depending on your performance in later ranked matches.